In one aspect, this invention relates to a poly(arylene sulfide) composite composition. In another aspect, this invention relates to process for forming a poly(arylene sulfide) composite composition. In yet another aspect, this invention relates to incorporating additives into a fiber-reinforced poly(arylene sulfide) composition.
Reinforced poly(arylene sulfide) compositions are highly suitable for forming into a variety of objects. Fibers are generally used to reinforce such compositions. Fibers commonly used for glass fibers, carbon fibers, and aramid fibers. Some fibers present special problems because they exhibit anisotropic thermal expansion. With increasing temperatures, carbon fibers for example expand negligibly in the longitudinal direction but have substantial diametrical expansion. When carbon fibers are utilized to reinforce poly(arylene sulfide) compositions, temperature cycling causes microcracking in the resin matrix because of the differing temperature expansion between the carbon fiber and the resin matrix. This microcracking is noticed particularly in cross ply, quasi-isotropic and fabric laminates since stresses are set up between adjacent groups of fibers which are nonparallel.
Where long carbon fibers are used to reinforce a poly(arylene sulfide) matrix, microcracks extending from the carbon fibers into the matrix provide an avenue for liquid absorption due to capillary action which can cause further deterioration in composite strength.
The mechanism of crack formation in long carbon fiber reinforced poly(arylene sulfide) compositions is different from crack formation in poly(arylene sulfide) injection molding compounds which can be reinforced with short fibers, generally having a length of 1 cm or less. In injection molding compounds, the cracks are caused because poly(arylene sulfide) resins shrink upon solidification and solidification occurs first on the outside surface of injection molded objects. The resin on the inside of an injection molded object solidifies last and occupies a lesser volume than is required to fill the volume previously determined by the outside surfaces of the object. The solidifying innermost resin pulls apart from the previously solidified outermost resin, resulting in cracks. The problem is especially acute in thick walled injection molded objects having a wall thickness of greater than 1 cm, for example.
On the other hand, composite lay-ups formed from a poly(arylene sulfide), such as polyphenylene sulfide (PPS) and long carbon fiber unidirectional prepreg display microcracking, particularly in cross ply and quasi-isotropic lay-ups. These microcracks are observable by cutting the laminate, polishing the cut surface and observing the surface microscopically by direct reflected light. The microcracks are typically up to a few microns wide and extend through the thickness of the ply. This may cause reduced strength in the composite and/or increased sensitivity to property-deteriorating fluids.